1. Field of the Invention
The present invention relates generally to the field of motor vehicle steering control apparatus, and more particularly to motor vehicle front wheel steering stabilization and return-to-center apparatus.
2. Discussion of the Prior Art
Many problems have been, and still are, related to motor vehicle steering gear which converts rotational steering wheel movement into side-to-side swiveling of the steerable wheels, thereby enabling a driver to control direction of vehicle travel.
An important motor vehicle safety requirement is that the vehicle's steering gear enable the driver to maintain steering control of the vehicle, regardless of road conditions which tend to oppose driver control through direct interaction with the wheels. Safe vehicle operation also dictates that vehicle turning be quickly and accurately responsive to steering wheel movement, without under- or over-steering, with no steering dead zones and with vehicle turning proportional to steering wheel turning, all with uniform, easy steering wheel turning forces being provided throughout the vehicle turning range.
Another important vehicle safety requirement is that at any held steering wheel position, the vehicle should exhibit no tendency to wander; and that upon release of the steering wheel after a turn the vehicle, of its own accord, quickly resumes straight ahead travel. On uncrowned roads, the vehicle should travel straight ahead in a "hands off" condition, without veering to either side, unless specifically adjusted to do so, as may be required for some classes of vehicles.
In spite of these restrictions and requirements, the steering gear should be configured so that the driver retains a driving "feel" for the vehicle and a comfortable sense of vehicle responsiveness and control.
In addition to being extremely reliable in operation, the steering gear should, for cost considerations, be relatively simple and require comparatively low maintenance. And, since the steering operation depends upon tire-to-road frictional contact, the steering gear should not be a substantial factor in tire wear.
To these ends, early motor vehicles with nonindependent front wheel suspension used relatively simple linkage systems, generally operated by rack and pinion steering mechanisms, to pivot the steerable wheels in unison while maintaining, unlike horse drawn carriages, a fixed, non-pivoting axle.
More sophisticated steering gear was needed to meet subsequent steering requirement of heavier and faster motor vehicles with independent front wheel suspension. The steering gear of most motor vehicles now typically includes a steering column to which a driver operated steering wheel is attached and in lower regions of which a projecting arm, known as a pitman arm, is connected to a gear box to swing, generally from fore to aft as the steering wheel is turned.
At each steerable wheel, a "knuckle" assembly is provided which includes a wheel mounting spindle, means for attaching the assembly to the vehicle's suspension system, and a projecting steering knuckle arm or lever enabling swiveling of the knuckle, and hence the wheel, for steering purposes. Two tie rods, usually of equal length, are provided, each being pivotally connected, at a tie rod ball and socket end, to a corresponding one of the steering knuckle arms.
Opposite ends of the two tie rods are typically connected, in laterally spaced apart relationship, to intermediate regions of a transverse relay rod, one end of which is pivotally connected to the pitman arm. The other end of the relay rod is pivotally connected to an idler arm, which is, in turn, pivotally connected to the vehicle frame.
Fore-to-aft pivotal movement of the pitman arm, as the steering wheel is turned, is transmitted through the relay rod to the individual tie rods which, through the steering knuckle arms, cause corresponding side-to-side swiveling or steering of the steerable wheels.
The gear box which is connected between the steering wheel shaft and pitman arm provides a mechanical advantage in the range of about 10:1 to 30:1 to enable overcoming frictional resistance between the tires and road surfaces with reasonable driver effort. The mechanical advantage also substantially reduces tendency of the wheels, when encountering obstacles, road irregularities, etc., to steer the vehicle by pulling the steering wheel out of the driver's hands. As weight and maximum speed of vehicles have increased, some type of power steering, which substantially increases the mechanical advantage associated with manual steering, has been provided on a large percentage of vehicles.
Complete descriptions of modern vehicle steering gear construction and operation may be found, for example, in a book entitled "Automative Suspensions--Steering Alignment and Brakes", 5th Edition, by Walter Billet and Walter Alley, published by the American Technical Society, 1974.
Vehicle steering control, in accordance with such safety and driving requirements as mentioned above, is typically provided by various different static, angular adjustments of the steerable wheels. Most familiar of these wheel adjustments are caster, camber and toe-in; less familiar are steering axis inclination angle and toe-out in turns. Although description and effects of these various adjustments may be found in publications, for example, in the above referenced automotive book, several important aspects are summarized below.
Wheel caster is a measure of the fore-aft inclined angle at which the wheel steering knuckle is mounted to the wheel spindle. Such inclination is ordinarily such as to cause an axis through the knuckle pivot points to intersect the road surface ahead of a road surface intersection of a vertical centerline through the wheel, the caster in such case being considered positive. The separation between the two road surface intersection points is commonly referred to as the caster effect, and is responsible for the road-tire forces which cause directional wheel stability and tendency of the wheels to travel straight ahead.
Because the wheel spindle is attached to the steering knuckle, an effect of caster is that as the castered wheel is swiveled from side to side, the spindle sweeps through an inclined path. Thus, the front end of the vehicle is caused to rise and fall; the front end must be lifted as the wheel turns in and is lowered as the wheel turns out. Typically, for every 1.degree. of caster, the front end of the vehicle rises about 3/16 inch at maximum wheel turn in. With equal caster provided on both front wheels, there is a tendency of front end weight to urge both the wheel towards a straight-ahead direction, as is desirable for "hands-off" steering. However, it also necessitates high steering wheel turning forces; thus, for heavy vehicles with several degrees of wheel caster, power steering is usually required.
Steering axis inclination is defined as lateral or transverse tilt of the steering knuckle, as opposed to the described fore-aft caster. Ordinarily the steering axis inclination angle is set so that an extension of the knuckle axis intersects the road surface in a vertical plane through the tire. This provides easy wheel pivoting during a turn. Due to orientation of steering axis inclination, the vehicle front end must be lifted when the wheels are swiveled in either direction from straight ahead. Since steering axis inclination acts in combination with caster, more vehicle front end lifting is required, in a turn, from the turned in wheel, causing non-symmetrical wheel loading during turns.
Camber is defined as sideways tilting of the steerable wheels from vertical, being measured when the wheels are in a straight ahead position. Positive camber occurs when the wheel top is titled outwardly, and is usually provided. Negative camber occurs when the wheel top is inwardly titled. Since camber, in effect, makes a cone of the steerable wheels, the two steerable wheels try to swivel in opposition, causing the vehicle to travel in a straight line when the camber on each wheel is the same. However, with positive camber, outside portions (in respect to the turn) of the tire tread must travel faster than vehicle speed, thereby causing abrasive tread slipping and increased tire wear. From a tire wear standpoint.
Taken in combination with caster angle, the camber angle changes as the wheel swivels, the camber angle being reduced when the wheel is turned out and being increased when the wheel is turned in. This occurs even for zero camber with the wheels straight ahead. Because of caster-camber interaction, tire tread wear is increased and tire forces imparted by the road surface are non-symmetrical during turns, thereby tending to reduce steering control.
The point of intersection (of the knuckle center line, the wheel vertical center line and steering axis centerline) depends upon camber, steering axis inclination, tire radius and distance between the wheel and the steering knuckle pivot points. When the point of intersection is located below the road surface, the wheels tend to toe-out, when located above the road surface, the wheels tend to toe-in. If the point of intersection is just at the road surface under straight ahead conditions, the point of intersection, which tends to shift during turns, may, at some points, be above the road surface and at others be below the surface, alternatingly causing toe-in and toe-out tendencies, uneven wheel loading and steering control degredation.
Wheel toe-in occurs when front regions of the two front wheels are closer together than are rear portions thereof. Toe-out occurs when the front regions are further apart than rear regions. Because natural tendency of positive castered wheels is to try to turn out, and since tolerances and wear of steering gear pivot points tends to permit such toe-out, most front vehicles are provided with slight tie-in under static conditions, typically from about 1/16 to 1/8 inch. Any amount toe-in (or toe-out) however, causes increased tire wear since sideways tread slipping must occur as the tire rolls along a road surface.
Differences in angles, from straight-ahead, between the two front wheels during a turn is known as "toe-out on turn". This difference in angle, which is typically about 2-3 degrees, occurs because each wheel is at a different distance and is ordinarily on a different radius line from the center of turning point. Toe-out on turn, which causes tire dragging around turns, resulting in increased tread wear, is largely determined by vehicle configuration but is also affected by caster, camber, toe-in and steering inclination.
From the foregoing, it can be appreciated that the interaction among the mentioned adjustments of caster, camber, toe-in, etc. is complex, certain adjustments tending to provide enhanced steering control in some wheel positions but often reduced steering control in other wheel positions. Thus "fine turning" of all the adjustments is necessary to maximize steering control at all wheel positions. If one of the factors becomes out of adjustment, tendencies of the vehicle front wheels to "shimmy", for the vehicle to "wander" along the road or to pull to one side commonly occur.
In most of such out-of-adjustment situations, tire tread wear is particularly accelerated, in often characteristic wear patterns. However, these adjustments, even when properly made to maximize steering control, result in tire wear which is substantially increased, due to tire drag through turns, tread side angle slipping, etc., over that which would be expected to occur with zero amounts of adjustment and with the tires rolling without slipping, dragging, and the like.
Recognizing the relationship between the different mentioned wheel adjustments for steering control and accelerated tire wear, trade-offs are usually also necessary between steering control and tire wear when determining the adjustments to be made. Thus, some steering control must be sacrificed to enable reasonable, although far from maximum, tire life.
It is significant, also, that the tire tread slipping, over speeding, and the like, which is a result of front wheel adjustments to provide driving control and vehicle stability, actually tend to cause a dangerous reduction in steering control and stability in many turn conditions.
Coupled with the typically complex and often unpredictable interaction between the various wheel adjustment factors mentioned in relatively sharp turns at moderate or high speeds, the tread distortion, slipping and shifted centers of road engagement caused by these adjustments causes significant reduction of tire traction when traction is needed most. This effect of reduced tire tread traction is particularly dangerous if the road surface is wet, oily, icy or otherwise slippery.
Improved traction in such situation may often be achieved by use of wide, low, pressure tires which are in current common usage. However, with wider tires with more tread in pavement engagement, more unwanted tread movement as a result of caster, camber, etc. tends to occur. Thus, improvement in traction tends to be less than should be expected and tire wear tends to be more pronounced. In addition, gasoline economy tends to be decreased by use of wide low pressure tires which typically have increased frictional rolling contact with the pavement.
Attempts to provide auxillary vehicle steering control, typically in conjunction with existing caster, camber, toe-in, etc. adjustments have frequently been made. For example, in the several years preceeding about 1927 a number of United States patents were granted for add-on, spring-type apparatus for motor vehicle steering gear. These patents, most of which were intended to improve steering stabilization, are exemplified by those to Fitzgerald (U.S. Pat. No. 1,121,818), Miller (U.S. Pat. No. 1,144,771), Huffman (U.S. Pat. No. 1,378,542), Pewther (U.S. Pat. Nos. 1,519,046 and 1,569,018), Cambell (U.S. Pat. No. 1,577,821), Stevens (No. 1,648,627) and Cavanagh (1,653,944).
Such patents typically disclose springs disposed between different pairs of steering gear elements, frequently for taking up manufacturing tolerances or wear in pivot connections between the elements and thereby reduce "play" in the steering system.
The number and general similarity of patents in this general group may be explainable by the fact that automobiles had, by the 1920's became more prevalant, their speed capabilities and size had increased, and better performance was expected, although roads had not yet been universally paved.
Also, by that time period, more women were driving and easier driving control was necessary to be consistent with general lesser strength of women drivers. A common tendency of cars of that era on rough, unpaved roads, was that when frequent ruts, potholes, rocks, etc., were hit by the front wheels, the steering wheel was frequently voluntarily torn from the drivers grasp, often with resulting injury to the driver.
With more universal road macadamizing or paving in this country occurring during the late 1920's, any popularity or, possibly, necessity of such auxillary, spring-type apparatus appears to have faded, and no further patents appear to have been granted on such apparatus, until the late 1960's when similar patents were granted, for example, to Leggett (U.S. Pat. No. 3,448,991) and Hefren (U.S. Pat. Nos. 3,848,845 and 3,980,315).
A common deficiency, however, of all such auxillary, spring-type add-on apparatus know to the applicant for improving steering stability of motor vehicles is that the apparatus were designed for improving vehicle steering control which was already provided to some extent by conventional caster, camber, toe-in, etc. adjustments. Although steering control and vehicle stability might be improved by installation of the add-on apparatus, the basic problems of steering instability by irregular and slipping tire engagement with the road surface during turns and excessive tire wear were generally not addressed or solved.
To overcome these and other problems, applicant has invented programmable-type vehicle steering control apparatus for adding onto preexisting steering gear, the apparatus providing for superior steering control while eliminating caster, camber, toe-in and steering axis inclination.
As a result, the steerable wheels of vehicles on which applicant's apparatus is installed roll freely through turns with virtually no tire tread distortion or slipping and with consequent greatly improved tread-to-road surface engagement and traction. Because lifting of the front end during turns is eliminated by elimination of wheel caster, driver steering forces are substantially reduced throughout the vehicle turning range and power steering, with its attendant costs and weight, is ordinarily unnecessary and can be eliminated.
Vehicle gas mileage is improved by applicant's steering control apparatus not only by reduction of front tire frictional losses but also by enabling higher air pressure in the front tires without sacrificing steering control and ride quality. As a result, tire life is also expected to be greatly improved, typically by as much as 30-50 percent, with no adverse side effects.
Additionally, front end and vehicle life is expected to be significantly increased due to reduced shock and vibration. Battery life is also expected to be improved, due to less oxide being broken loose from the battery plates by shock and vibration.